This disclosure relates generally to tracking systems that use magnetic fields to determine positions and orientations of an object, such as systems used for tracking instruments and devices during surgical interventions and other medical procedures. More particularly, this disclosure relates to a system and method to more accurately determine position and orientation of an object.
Tracking systems have been used in various industries and applications to provide position information relating to objects. For example, electromagnetic tracking may be useful in aviation applications, motion sensing applications, and medical applications. In medical applications, tracking systems have been used to provide an operator (e.g., a physician) with information to assist in the precise and rapid positioning of a medical device located in or near a patient's body. In general, an image may be displayed on a monitor to provide positioning information to an operator. The image may include a visualization of the patient's anatomy with an icon on the image representing the device. As the device is positioned with respect to the patient's body, the displayed image is updated to reflect the correct device coordinates. The base image of the patient's anatomy may be generated either prior to, or during, the medical procedure. For example, any suitable medical imaging technique, such as X-ray, computed tomography (CT), magnetic resonance imaging (MRI), positron emission tomography (PET), and ultrasound, may be utilized to provide the base image displayed during tracking. The combination of the base image and the representation of the tracked device provide positioning information that allows a medical practitioner to manipulate a device to a desired position and/or associate information gathered to a precise location.
To determine device location, tracking systems may utilize a method of electromagnetic (EM) field generation and detection. Using this method, at least one magnetic field is generated from one or more EM sensors, and the magnetic fields are detected by one or more complementary EM sensors. In such a system the mutual inductance of the EM field detected may be processed to resolve a position and/or orientation of the EM sensors relative to one another. For example, an EM sensor may be fixed in a known position, with a complementary EM sensor mounted at the operative end of a device. While the EM sensor generates a magnetic field, the magnetic field characteristics may be detected by the complementary EM sensor. The detected characteristics may be processed to determine the position and orientation (e.g., the X, Y and Z coordinates, as well as the roll, pitch and yaw angles) of the EM sensors relative to one another.
To provide for more accurate device tracking, various arrangements of EM sensors around a tracking area have been used. For example, four EM sensors may be located at the corners of a rectangular region. In this configuration, each of the four EM sensors may generate a magnetic field that is sensed by a complementary EM sensor. A signal indicative of the detected magnetic field characteristic may then be transmitted to a processor. On receipt of the signal, the processor may consider the mutual inductance of each magnetic field sensed to estimate the distance between each EM sensor and each complementary EM sensor. By triangulation, the position and/or orientation of the sensors may be estimated. Other configurations may provide a multitude of EM sensors located about the corners of a cubic volume wherein a complementary EM sensor is tracked in the volume. Although these methods may provide sufficient accuracy, there are several instances in which the sensors located about the periphery do not provide a sufficient estimate of position and/or orientation. For example, where four EM sensors are located at the corners of a rectangular region, the accuracy may vary depending on the location of the complementary sensor being tracked. In this configuration, if the complementary EM sensor is located a significant distance from the plane where four EM sensors are located, the mutual inductance sensed and processed may provide a sufficient position estimate. However, as the complementary EM sensor approaches the plane where the four EM sensors are located, a minimal change in the distance from the plane results in a minimal difference in the mutual inductance sensed between the EM sensors. Accordingly, the small variations in the magnetic field make it difficult for processing to accurately resolve the position of the complementary EM sensors in a direction normal the plane of the four EM sensors.
Accordingly, there is a desire to provide an electromagnetic field tracking system, wherein EM sensors are configured to provide for processing that may accurately determine position and/or orientation of a device.